Chapter 5: Cloud Development and Precipitation Atmospheric Stability Atmospheric Stability Determining stability Determining stability Cloud development and stability Cloud development and stability Precipitation processes Precipitation processes Precipitation types Precipitation types Measuring precipitation Measuring precipitation

Atmospheric Stability Q: Why does the air rise on some occasions and not on others? Q: Why do the size and shape of clouds vary so much when the air does rise? A: because of different atmospheric stability conditions Stable and unstable equilibrium stability analysis: giving initial perturbations, Stable: parcel moves back Unstable: parcel moves away from the original position

Atmospheric Stability adiabatic process: no heat exchange of the air parcel with the environment so that rising air parcel expands and cools adiabatic process: no heat exchange of the air parcel with the environment so that rising air parcel expands and cools dry adiabatic lapse rate for parcels (10 C/km) dry adiabatic lapse rate for parcels (10 C/km) moist adiabatic lapse rate for parcels (taken as 6 C/km) moist adiabatic lapse rate for parcels (taken as 6 C/km) environmental lapse rate for the atmosphere (~ 6.5 C/km) environmental lapse rate for the atmosphere (~ 6.5 C/km) Q1: if a rising parcel’s T decreases at 10 C/km in an adiabatic process, does its T increases at 10 C/km as the parcel descends? a) yes, b) no Q2: why is the moist lapse rate lower than dry lapse rate? a) because condensation occurs in the moist adiabatic process b) because evaporation occurs in the dry adiabatic process c) because condensation occurs in the dry adiabatic process

Q3: why is the moist lapse rate lower over tropics (higher T) than over polar regions (lower T)? because warm saturated air contains more liquid water for condensation; because warm saturated air contains more liquid water for condensation; because cold saturated air contains more liquid water for condensation because cold saturated air contains more liquid water for condensation Q4: What does a radiosonde measure? dry adiabatic lapse rate; dry adiabatic lapse rate; moist lapse rate; moist lapse rate; environmental lapse rate environmental lapse rate

Determining Stability Stability analysis: assume a parcel rises following the dry or moist adiabatic process; then compare its T p with the environmental T e (following environmental lapse rate); colder T means denser air. Stable condition: If a rising parcel’s T p < T e, it is denser and would sink back. Unstable condition: If the rising parcel’s T p > T e, it is less dense and will continue to rise Stability does not control whether air will rise or sink. Rather, it controls whether rising air will continue to rise or whether sinking air will continue to sink. Stability does not control whether air will rise or sink. Rather, it controls whether rising air will continue to rise or whether sinking air will continue to sink.

A Stable Atmosphere stabilizing processes stabilizing processes nighttime surface radiational cooling; nighttime surface radiational cooling; warm air advected to cold surface; warm air advected to cold surface; air aloft warming (e.g., subsidence inversions) air aloft warming (e.g., subsidence inversions) Stable air provides idealStable air provides ideal conditions for high pollution levels. conditions for high pollution levels.

An Unstable Atmosphere destabilizing processes destabilizing processes daytime solar heating of surface air; daytime solar heating of surface air; cold air advected to warm surface cold air advected to warm surface superadiabatic lapse rates (> 10 C/km) superadiabatic lapse rates (> 10 C/km) Unstable air tends to be well-mixed.Unstable air tends to be well-mixed.

Conditionally Unstable Air Conditional instability: Conditional instability: environmental lapse rate between dry and moist lapse rates environmental lapse rate between dry and moist lapse rates Condensation level Condensation level cloud base cloud base Q5: exactly at what height in the figure would T p = T e ? a) above 2 km, b) at 2 km, c) at 1.33 km

Q6: If environmental lapse is greater than dry lapse rate, the atmosphere is a) stable, b) unstable, c) conditionally unstable Q7: If environmental lapse is less than moist lapse rate, the atmosphere is a) stable, b) unstable, c) conditionally unstable Q8: The earth’s atmosphere is ordinarily a) stable, b) unstable, c) conditionally unstable Q9: The air T in an unsaturated parcel follows the a) dry lapse rate, b) moist lapse rate, c) environmental lapse rate Q10: If air T increases with height, the air is surely stable. If it decreases with height, the air is: a) stable, b), unstable, c) undecided

Cloud Development and Stability Layered clouds tend to form in a stable atmosphere; Cumuliform clouds tend to form in an unstable or conditionally unstable atmosphere Q11: Do you usually expect to see layered clouds in Tucson during the day in summer? a) yes, b) no Q12: Do you usually expect to see cumulus clouds at night over midlatitudes? a) yes, b) no

Four Ways for Cumulus Development surface heating and free convection surface heating and free convection uplift along topography uplift along topography widespread ascent widespread ascent lifting along weather fronts lifting along weather fronts

Convection and Clouds thermals thermals fair weather cumulus fair weather cumulus Fair weather cumulus provide a visual marker of thermals.Fair weather cumulus provide a visual marker of thermals. Bases of fair-weather cumulus clouds marks the lifting condensation level, the level at which rising air first becomes saturated.Bases of fair-weather cumulus clouds marks the lifting condensation level, the level at which rising air first becomes saturated.

Topography and Clouds orographic uplift orographic uplift rain shadow rain shadow The rain shadow works for snow too. Due to frequent westerly winds, the western slope of the Rocky Mountains receives much more precipitation than the eastern slope.The rain shadow works for snow too. Due to frequent westerly winds, the western slope of the Rocky Mountains receives much more precipitation than the eastern slope.

Precipitation Processes Pay attention to the 2 orders of magnitude change in diameter Growth of cloud droplets by condensation is too slow (a few days), but rain drops can develop in < 1 hr in nature

Collision and Coalescence Process Warm clouds (with T above freezing) Warm clouds (with T above freezing) terminal velocity terminal velocity large drops fall faster than large drops fall faster than small drops small drops coalescence: coalescence: the merging of a large cloud the merging of a large cloud droplet with small droplets droplet with small droplets by collision by collision Q13: Do larger drops fall faster in a vacuum? a) yes, b) no

Ice Crystal Process cold clouds (ice crystals and liquid drop coexist) cold clouds (ice crystals and liquid drop coexist) supercooled water droplets due to lack of ice nuclei supercooled water droplets due to lack of ice nuclei

Saturation vapor pressures over liquid water is higher than over ice (see Fig. 4.5 insert on p. 83) Saturation vapor pressures over liquid water is higher than over ice (see Fig. 4.5 insert on p. 83) This causes water vapor molecules to diffuse from water droplet towards the ice crystals This causes water vapor molecules to diffuse from water droplet towards the ice crystals Ice crystals grow at the expense of water droplets Ice crystals grow at the expense of water droplets

Snow pellets and snowflakes Accretion: ice crystals grow by colliding with supercooled water droplets to form snow pellets Accretion: ice crystals grow by colliding with supercooled water droplets to form snow pellets

Cloud Seeding and Precipitation cloud seeding cloud seeding inject (or seed) a cloud with small particles that will act as inject (or seed) a cloud with small particles that will act as nuclei so that the cloud particles will grow large enough to nuclei so that the cloud particles will grow large enough to fall to the surface as precipitation fall to the surface as precipitation silver iodide silver iodide as cloud-seeding agent because it has a crystalline structure similar to an ice crystal as cloud-seeding agent because it has a crystalline structure similar to an ice crystal Very popular in some countries Very popular in some countries It is very difficult to determine whether a cloud seeding attempt is successful. How would you know whether the cloud would have resulted in precipitation if it hadn’t been seeded?It is very difficult to determine whether a cloud seeding attempt is successful. How would you know whether the cloud would have resulted in precipitation if it hadn’t been seeded?

Q14: In warm clouds (with T above freezing), cloud droplets grow to rain droplets primarily through a)collision/coalescence process b)Ice crystal process c)Accretion Q15: Cloud liquid droplet collision is called a)coalescence, b) accretion Q16: What is the purpose of using silver iodide for the seeding of supercooled clouds? a)Increase the number of ice nuclei b)Increase the number of cloud nuclai

Precipitation Type: Rain Rain: drop diameter of 0.5 ~ 6mm Rain: drop diameter of 0.5 ~ 6mm Drizzle: drop diameter < 0.5mm Drizzle: drop diameter < 0.5mm Virga: rainfall not reaching surface Virga: rainfall not reaching surface Rain drop shape Rain drop shape small to large droplet medium (> 2 mm diameter)

Snow Snow Snow Fallstreaks: ice crystals and snowflakes from high cirrus clouds that usually do not reach surface Fallstreaks: ice crystals and snowflakes from high cirrus clouds that usually do not reach surface Dendrite: Snowflake shape depends on both temperature and relative humidity Dendrite: Snowflake shape depends on both temperature and relative humidity Blizzard: low T and strong wind bearing large amounts of snow, reducing visibility to a few meters Blizzard: low T and strong wind bearing large amounts of snow, reducing visibility to a few meters Flurries: snow falling from cumulus clouds Flurries: snow falling from cumulus clouds

Sleet and Freezing Rain Sleet: frozen raindrop; makes a ‘tap tap’ sound when falling on glass Sleet: frozen raindrop; makes a ‘tap tap’ sound when falling on glass Freezing rain: supercooled liquid drops spread out and freeze on cold surface Freezing rain: supercooled liquid drops spread out and freeze on cold surface Rime: white granular ice Rime: white granular ice

Snow Grains and Snow Pellets snow grains: snow equivalent of drizzle snow grains: snow equivalent of drizzle snow pellets: larger and bounce on surface; formed snow pellets: larger and bounce on surface; formed as ice crystals collide with supercooled water droplets as ice crystals collide with supercooled water droplets Graupel: Graupel: when snow pellets accumulate a heavy coating of rime, when snow pellets accumulate a heavy coating of rime, they are called graupel they are called graupel

Hail updraft cycles updraft cycles accretion accretion A hailstone can be sliced open to reveal accretion rings, one for each updraft cycle.A hailstone can be sliced open to reveal accretion rings, one for each updraft cycle.

Q17: what is the shape of a small rain droplet? a) tear, b) spherical, c) mushroom Q18: What is the difference between sleet and freezing rain? supercooled liquid drops spreading out and freezing on cold surface (ground, trees, …) and substantially affect driving conditions (both on the road and on the wind shield) A: sleet refers to raindrops freezing through a deep cold layer below cloud and could bounce on the ground, while freezing rain refers to supercooled liquid drops spreading out and freezing on cold surface (ground, trees, …) and substantially affect driving conditions (both on the road and on the wind shield) Q19: When ice crystals collide with supercooled water droplets, what would be formed? a) snow pellets, b) graupel, c) hail a) snow pellets, b) graupel, c) hail

Precipitation Measurement Instruments standard rain gauge: 0.01 inch interval ; trace standard rain gauge: 0.01 inch interval ; trace tipping bucket rain gauge: used in ASOS tipping bucket rain gauge: used in ASOS difficult to capture rain in a bucket when wind blows strongly.difficult to capture rain in a bucket when wind blows strongly. Tipping bucket underestimates rainfall for heavy eventsTipping bucket underestimates rainfall for heavy events

Doppler Radar and Precipitation Radar: radio detection and ranging Radar: radio detection and ranging Doppler radar: Doppler radar: use Doppler shift use Doppler shift (e.g., a higher-pitched (e.g., a higher-pitched whistle as a train approaches you); Provide precipitation area and intensity; Provide horizontal speed of falling rain

Q20: Can you claim from your raingauge that precipitaiton rate is in/hour? a) yes, b) no